In high-voltage electric equipment and its components, electric insulation systems are often applied, in which there are placed electrically conductive elements used for shaping the electric field generated by live elements of that equipment or its components. The elements used for shaping the field, placed in insulating material, have typically a form of screens determining the appropriate distribution of the electric field. The appropriate distribution of the electric field is especially important in the construction and operation of high-voltage bushings, high-voltage cables, cable accessories and connection conductors in measuring equipment such as current, voltage or combined instrument transformers. Screens for shaping electric fields usually have the form of conductive sheets placed between layers of insulating material. These sheets are most frequently made of aluminum foil. Sometimes conductive screens made of conductive paper or of fabric that conducts electric current are also used. The insulation material, most frequently made of sheets of insulation paper, is wound together with the conductive screens around a conductor, and then it is impregnated with insulation material in the form of transformer oil or hardenable resin. Usually, before impregnation, the insulating material undergoes a drying process.
In order to obtain an even distribution of the electric field in the cross-section of the insulation structure, conditions ensuring an appropriate voltage distribution among all conductive screens and the maintenance of a constant electric potential throughout the whole surface of each individual screen should be fulfilled. These conditions depend upon the electric capacitance between the screens, the dimensions of the individual screens, their electric impedance, in particular electric resistance, and the maximum frequency at which the appropriate shaping of the electric field is required.
From a British patent description No. GB 991546 there is known a high-voltage insulation structure intended for insulating high-voltage equipment, which contains an insulation skeleton consisting of layers of insulating sheets which are so formed that the overall thickness of a layer is many times the thickness of the basic sheet material and that the sheet material occupies only a fraction of the overall volume of the interior of the skeleton. The space between the insulating and the conductive sheets is filled with a dielectric material. Insulating sheets are made of an absorption material such as paper, which can easily be impregnated with oil or other fluid, or of a non-absorbing material such as a polymer material. Between the insulating layers there are placed conductive sheets, for instance in the form of conductive foil supported on the crests of the corrugations of the corrugated insulating sheet forming the insulation skeleton.
Conductive sheets in the form of metal foil which is placed between layers of an insulating material are widely applied in electric insulating structures. Examples of the use of such insulating structures in various designs of high-voltage bushings are presented in the following patent descriptions: U.S. Pat. Nos. 3,875,327, 4,362,897, 4,338,487, 4,387,266, 4,500,745 and GB 1 125 964.
From a Japanese patent description JP 01283716 there is known a cast bushing in which the conductive sheets are made of fabric or nonwoven cloth having a conductive layer on its surface, e.g. in the form of conductive paint.
Another type of high-voltage insulation is known from application WO2006/001724. In the presented solution a high-voltage bushing is formed by winding layers of electrical insulation material around a cylindrical core. Sheets of conductive material, used for shaping the electric field in the bushing, are placed between those layers. At least one sheet of the conductive material is a structure made on the basis of paper, fabric or nonwoven cloth and it contains conducting particles suspended in it and forming a percolating network, electrically conducting in the sheet plane. The conducting particles have basically an elongated shape and such dimensions that the proportion of their length to the largest crosswise dimension is more than 10.
Metal sheets used as screens for shaping the electric field in high-voltage components in which epoxy resin is used as the insulating material, due to the difference between the coefficients of thermal expansion of metal foil and epoxy resin, cause mechanical stresses that are generated during the process of resin hardening. These stresses persist also after the end of the production process and they manifest themselves especially when such components are operated in very low temperatures.
Conducting sheets made of metal foil are characterized by a typically very high electric conductance. This property in connection with the geometrical arrangement of the sheets in the whole insulation system can result in a generation in that system of electromagnetic resonant oscillations of high frequencies and very large quality factor. Resonance oscillations excited in such systems can cause a local overvoltage leading to insulation damage. Excitation sources triggering such oscillations can emerge in systems comprising semiconductor converters that generate high frequencies, such as systems used in DC voltage transmission, in wind power plants, or in the industrial power systems.
On the other hand, the poor conductivity of paints typically based on carbon materials causes limitations in the use of conducting sheets in the form of such paints in high-voltage equipment, especially in applications in which shaping of the electric field is required for relatively fast transients, such as a lightning impulse or a chopped wave.
Application of conducting sheets made of materials containing conductive particles causes the risk of such particles being released during the process of cutting into suitably shaped sheets. Penetration of such particles into the insulating material can weaken the dielectric properties of the insulating structure.